Make PostOffice more idiomatic to use as a library

[mpalmer]

My use case is that I've got a Ruby program that does end-to-end testing on a system that (amongst many other things) sends email. So for each test, I want to spin up an SMTP server (on a random high port), configure the system under test to use said SMTP server, and then validate the emails sent.

PostOffice is basically perfect for this -- it's small, easy-to-understand code, no massive external dependencies, quick and easy to spin up a SMTP server in-process, receive mail, grab the message out of the server, and assert the needful. I only needed to make a few tiny tweaks to be perfect for my use-case. I can't easily verify that I haven't broken anything (other than the bits I use), but I did try to keep the changes small and self-contained.

In detail, the changes I made are:

• Move all top-level identifiers into a PostOffice module, to give a common namespace and avoid collisions (which includes moving the files, for include purposes);
• Remove the global $log variable, and instead pass a logger where its needed;
• Separate the initialization and operation of the SMTPServer class, because -- since I'm using the SMTP server in tests -- I need random-free-port allocation, and I need to be able to find out what port was assigned (so I can configure the system under test to use that port). So as its done now, I can setup the server on the main thread, get the port number, and then set the server running in a separate thread.
• Received messages are now stored in the server object, as well as in the global singleton Store. This is needed because I'm running multiple tests in parallel, and so having a single place where mails are received into would end badly.
• I taught the RSET command handler to send a response, because it turns out the SMTP client library my system is using actually uses that command, and expects a response before completing the mail sending, which took a few minutes to figure out...

I think that's about it. Hopefully the changes are appropriate for inclusion in the official gem.